Pneumatic tire

ABSTRACT

A tire has a cap rubber formed of nonconductive rubber and forms a ground surface, and a conductive portion provided in at least one side end portion of a pair of side end portions in both ends in a tire width direction. The conductive portion is formed of a conductive rubber, and reaches a side surface of the cap rubber from the ground surface through an inner portion of the cap rubber. The conductive portion is relatively small in a thickness in the ground surface and a side surface of the cap rubber, is relatively large in the maximum thickness of an intermediate portion between the ground surface and the side surface of the cap rubber, and is formed into a crescent shape which is curved into an inner side in a tire width direction and an inner side in a radial direction, in a tire meridian cross section.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to a pneumatic tire which can dischargestatic electricity generated in a vehicle body and the tire to a roadsurface.

Description of the Related Art

In recent years, for the purpose of reducing a rolling resistance of atire which has strong relationship to a fuel consumption performance,there has been proposed a pneumatic tire in which a rubber member suchas a tread rubber is formed by a non-conductive rubber blended withsilica at a high rate. However, since an electric resistance is higherin the rubber member in comparison with a conventional product which isformed by a conductive rubber blended with carbon black at a high rate,and inhibits static electricity generated in a vehicle body or the tirefrom being discharged to a road surface, the rubber member has a problemthat a problem such as a radio noise tends to be generated.Consequently, it is necessary to appropriately secure a conductive routefor discharging the static electricity.

Then, there has been developed a pneumatic tire structured such that acurrent-carrying performance can be achieved by providing a conductiverubber where a carbon black is blended, while forming a tread rubber bya nonconductive rubber. For example, in a pneumatic tire described inJP-A-2013-95323, there is disclosed provision of a branched conductiveportion extending to a side surface or a bottom surface of a cap rubberfrom a ground surface in both end portions in a tire width direction ofthe cap rubber formed by a nonconductive rubber. There is a descriptionthat not only a conductive path can be secured, but also an effect ofreducing noise can be achieved by the conductive portion, according tothe structure.

SUMMARY OF THE INVENTION

The present disclosure is made by taking the above circumstances intoconsideration, and an object of the present disclosure is to provide apneumatic tire having a conductive portion which achieves otherfunctions than the conductive path.

The present disclosure employs the following means for achieving theobject.

In other words, according to the present disclosure, there is provided apneumatic tire including a cap rubber which is formed of a nonconductiverubber and forms a ground surface and a conductive portion which isprovided in at least one side end portion of a pair of side end portionsin both ends in a tire width direction of the cap rubber. The conductiveportion is formed of a conductive rubber, and reaches a side surface ofthe cap rubber from the ground surface through an inner portion of thecap rubber. The conductive portion is relatively small in a thickness inthe ground surface and a side surface of the cap rubber, is relativelylarge in the maximum thickness of an intermediate portion between theground surface and the side surface of the cap rubber, and is formedinto a crescent shape which is curved into an inner side in a tire widthdirection and an inner side in a radial direction, in a tire meridiancross section.

According to the structure, since the conductive portion is famed intothe crescent shape which is curved into the inner side in the tire widthdirection and the inner side in the radial direction, and is relativelylarge in the maximum thickness of the intermediate portion in comparisonwith the end portion, the deformation extending in the tire radialdirection is suppressed in comparison with the case that the thicknessis fixed. Since the strain is reduced by the suppression of thedefamation, the durability is improved. Further, the rolling resistanceis reduced by the suppression of the deformation. Further, since theconductive portion supports in relation to the radial direction, thebraking performance is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a tire meridian cross sectional view showing an example of apneumatic tire according to the present disclosure;

FIG. 2 is a cross sectional view schematically showing a tread rubberbefore being vulcanized;

FIG. 3 is a cross sectional view schematically showing an area ratiobetween a cap rubber and a conductive portion before being vulcanized;

FIG. 4A is a cross sectional view showing a shoulder portion after beingvulcanized;

FIG. 4B is a cross sectional view showing a shoulder portion after beingvulcanized according to the other embodiment;

FIG. 5A is a cross sectional view showing a shoulder portion after beingvulcanized according to the other embodiment than the above;

FIG. 5B is a cross sectional view showing a shoulder portion after beingvulcanized according to the other embodiment than the above;

FIG. 6A is a cross sectional view schematically showing a tread rubberbefore being vulcanized according to a comparative example 1; and

FIG. 6B is a cross sectional view schematically showing a tread rubberbefore being vulcanized according to a comparative example 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will be given below of a pneumatic tire according to anembodiment of the present disclosure with reference to the accompanyingdrawings.

As shown in FIG. 1, a pneumatic tire T is provided with a pair of beadportions 1, side wall portions 2 which extend to outer sides in a tireradial direction RD from the respective bead portions 1, and a treadportion 3 which is connected to outside ends in the tire radialdirection RD from both the side wall portions 2. An annular bead core 1a and a bead filler 1 b are arranged in the bead portion 1, the annularbead core 1 a covering a convergence body such as a steel wire by arubber, and the bead filler 1 b being made of a hard rubber.

Further, the tire T is provided with a toroidal carcass layer 4 whichruns into the bead portions 1 from the tread portion 3 via the side wallportions 2. The carcass layer 4 is provided between a pair of beadportions 1, is constructed by at least one carcass ply, and is locked ina state in which its end portions are rolled up via the bead cores 1 a.The carcass ply is formed by coating with a topping rubber a cord whichextends approximately vertically to a tire equator CL. An inner linerrubber 4 a for retaining a pneumatic pressure is arranged in an innerside of the carcass layer 4.

Further, a side wall rubber 6 is provided in an outer side of thecarcass layer 4 in the side wall portion 2. Further, a rim strip rubber7 is provided in an outer side of the carcass layer 4 in the beadportion 1, the rim strip rubber 7 coming into contact with a rim (notshown) when being installed to the rim. In the present embodiment, thetopping rubber of the carcass layer 4 and the rim strip rubber 7 arefamed of a conductive rubber, and the side wall rubber 6 is formed of anonconductive rubber.

An outer side of the carcass layer 4 in the tread portion 3 is providedwith a belt 4 b for reinforcing the carcass layer 4, a bet reinforcingmember 4 c, and a tread rubber 5 in this order from an inner side towardan outer side. The belt 4 b is constructed by a plurality of belt plies.The belt reinforcing member 4 b is constructed by coating a cordextending in a tire peripheral direction with a topping rubber. The beltreinforcing member 4 b may be omitted as occasion demands.

As shown in FIGS. 1 and 2, the tread rubber 5 has a cap rubber 50 whichis formed of the nonconductive rubber and constructs a ground surface E,a base rubber 51 which is provided in an inner side in a tire radialdirection of the cap rubber 50, and a conductive portion 52 which isformed of the conductive rubber and reaches a side surface 50 a of thecap rubber 50 from the ground surface E. A plurality of main grooves 5 aextending along a tire circumferential direction are formed on a surfaceof the cap rubber 50. The main grooves 5 a are provided with a TreadWear Indicator (TWI) which is a projection protruding out of a groovebottom. The TWI indicates a tire replacement time due to wear of thetire. In the present embodiment, the base rubber 51 is famed of theconductive rubber, however, may be formed of the nonconductive rubber.

In the above, the ground surface is a surface which is grounded onto aroad surface when the tire is vertically put on a flat road surface in astate in which the tire is assembled in a normal rim, and a normalinternal pressure is filled, and a normal load is applied to the tire,and an outermost position in the tire width direction WD comes to aground end E. The normal load and the normal internal pressure indicatea maximum load (a design normal load in the case of a tire for apassenger car) which is defined in JISD4202 (specification of anautomotive tire) and a corresponding pneumatic pressure, and the normalrim indicates a standard rim which is defined in JISD4202 in principle.

The present embodiment employs a side-on tread structure achieved bymounting the side wall rubbers 6 onto both side end portions of thetread rubber 5, however, can employ a tread-on side structure achievedby mounting both side end portions of the tread rubber onto outer endsin the tire radial direction RD of the side wall rubbers, without beinglimited to the side-on tread structure.

Here, the conductive rubber is exemplified by a rubber in which a volumeresistivity indicates a value less than 10⁸ Ω·cm, and is produced, forexample, by blending a carbon black serving as a reinforcing agent in araw material rubber at a high rate. The conductive rubber can beobtained by blending a known conductivity applying agent, for example, acarbon-based conductivity applying agent such as a carbon fiber or agraphite, and a metal-based conductivity applying agent such as a metalpowder, a metal oxide, a metal flake or a metal fiber, in addition tothe carbon black.

Further, the non-conductive rubber is exemplified by a rubber in which avolume resistivity indicates a value equal to or more than 10⁸ Ω·cm, andis exemplified by a material obtained by blending a silica serving as areinforcing agent in the raw material rubber at a high rate. The silicais blended, for example, at 30 to 100 weight part in relation to 100weight part of the raw material rubber component. The silica preferablyemploys a wet silica, however, can use any silica which is generallyused as the reinforcing agent, without limitation. The non-conductiverubber may be produced by blending a burned clay, a hard clay, or acalcium carbonate, in addition to the silica such as a precipitatedsilica or a silicic anhydride.

As the raw material rubber mentioned above, a natural rubber, a styrenebutadiene rubber (SBR), a butadiene rubber (BR), an isoprene rubber (IR)and an isobutylene-isoprene rubber (IIR) can be listed up, and they areused respectively by itself or by mixing two or more kinds. Avulcanizing agent, a vulcanization accelerator, a plasticizer or anantioxidant is appropriately blended in the raw material rubber.

In the light of enhancing a durability and improving a conductionperformance, the conductive rubber desirably has a composition that anitrogen adsorption specific surface area: N₂SA (m²/g)×compositionamount (mass %) of carbon black is equal to or more than 1900,preferably equal to or more than 2000, and a dibutyl phthalate oilabsorption: DBP (ml/100 g)×composition amount (mass %) of carbon blackis equal to or more than 1500, preferably equal to or more than 1700.N₂SA can be determined in conformity to ASTM D3037-89, and DBP can bedetermined in conformity to D2414-90.

FIG. 2 schematically shows the tread rubber 5 before being vulcanized.As shown in FIGS. 1 and 2, the conductive portion 52 is provided in atleast one side end portion of a pair of side end portions in both sidesin the tire width direction of the cap rubber 50. In the presentembodiment, the conductive portion 52 is provided only in the side endportion which is an installation inner side (IN) to the vehicle. Ofcourse, the conductive portion 52 may be provided in both of theinstallation inner side and an installation outer side, or may beprovided only in the installation outer side. The conductive portion 52is preferably provided at least in the installation inner side (IN). Theinstallation inner side (IN) has larger load due to a camber angle incomparison with the installation outer side and tends to ground, so thatan effect of the conductive portion 52 mentioned later tends to appear.A tire outer surface (particularly the side wall) is provided with adisplay which indicates an installing direction of the tire to thevehicle. The display “IN” is applied to the installation inner side tothe vehicle, and the display “OUT” is applied to the installation outerside.

The conductive portion 52 is formed of the conductive rubber, andreaches the side surface 50 a of the cap rubber 50 from the groundsurface E through an inner portion of the cap rubber 50. The conductiveportion 52 is relatively small in thicknesses D1 and D2 in the groundsurface E and the side surface 50 a of the cap rubber 50 and isrelatively large in a thickness D3 of an intermediate portion betweenthe ground surface E and the side surface 50 a of the cap rubber 50, ina tire meridian cross section. The conductive portion 52 is famed into acrescent shape which is curved into an inner side WD1 in the tire widthdirection and an inner side RD1 in the radial direction, in the tiremeridian cross section. The conductive portion 52 does not have anybranched structure, but has one crescent shape.

The side end portion of the cap rubber 50 corresponds to a shoulderportion in the tire, and tends to deform so as to extend toward theouter side in the tire radial direction when coming away from the roadsurface. Since the conductive portion 52 is famed into a crescent shapewhich is curved into the inner side WD1 in the tire width direction andthe inner side RD1 in the radial direction and the maximum thickness ofthe intermediate portion is relatively large in comparison with the endportion, a deformation extending in the tire radial direction issuppressed in comparison with the case that the thickness is fixed.Since a strain is reduced by the suppression of the defamation, adurability is improved. Further, a rolling resistance is reduced by thesuppression of the deformation. On the contrary, in the curved structurehaving the fixed thickness as JP-A-2013-95323, the conductive portion 52is not supported and tends to be deformed. As a result, the structure isthought to have no effect of reducing the rolling resistance. Further,since the support by the conductive portion 52 is effected in the radialdirection, a braking performance is improved.

Further, since the conductive portion 52 is relatively large in thethickness of the intermediate portion in comparison with the endportion, a width of the conductive portion 52 appearing on the roadsurface in the medium term of the wear becomes larger than that of abrand-new tire, and it is possible to improve a discharge effect.

Further, since the conductive portion 52 is formed into the curvedshape, a contact area with the cap rubber 50 is increased in comparisonwith a linear shape, an adhesion property is improved and the durabilitycan be improved.

In a particular shape, a relationship D1<D3 and D2<D3 is satisfied. Athickness D1 of the conductive portion 52 in the ground surface E is setto 0.1 to 0.5 mm. Since the thickness D1 is set to be equal to or morethan 0.1 mm, a current-carrying performance can be easily secured.Further, since the thickness D1 is equal to or less than 0.5 mm, it ispossible to better achieve the effect of reducing the rolling resistanceand the effect of improving the wet braking performance by suppressing avolume of the conductive rubber.

The thickness D2 of the conductive portion 52 in the side surface 50 aof the cap rubber 50 is set to 0.1 to 0.4 mm. Since the thickness D2 isequal to or more than 0.1 mm, the current-carrying performance can beeasily secured. Further, since the thickness is equal to or less than0.4 mm, it is possible to better achieve the effect of reducing therolling resistance.

The maximum thickness D3 of the intermediate portion between the groundsurface E and the side surface 50 a of the cap rubber 50 is set to 0.6to 1.5 mm. Since the maximum thickness D3 is equal to or more than 0.6mm, the durability and the braking performance can be easily secured.Further, since the thickness is equal to or less than 1.5 mm, it ispossible to better achieve the effect of reducing the rolling resistanceand the effect of improving the wet braking performance. The maximumthickness D3 is a maximum value of thicknesses in a normal linedirection in the inner side of the crescent shape.

Further, a relationship R1>R2 is satisfied in an average radius ofcurvature R1 in the inner side of the crescent shape, and an averageradius of curvature R2 in the outer side of the crescent shape. In thepresent specification, “average radius of curvature” means a radius ofcurvature of a curved line in the case that the curved surface isconstructed by a single curved line, and means an average value of theradii of curvature of curved lines in the case that the curved surfaceis constructed by a plurality of curved lines. In this shape, theaverage radius of curvature R1 is set to 5 to 45 mm, and the averageradius of curvature R2 is set to 25 to 65 mm.

The position of the conductive portion 52 in the side surface 50 a ofthe cap rubber 50 is essentially disposed on an imaginary extension ofthe TWI to the tire width direction or in an inner side in the radialdirection than the imaginary extension for securing the current-carryingperformance. Since the volume of the conductive portion 52 is increasedby extending to the inner side in the radial direction too much, theposition is preferably disposed on the imaginary extension forsuppressing the volume of the conductive portion 52.

As shown in FIG. 3, in a half of the tire meridian cross section, across sectional area S₁ (a diagonally left-up hatching area in FIG. 3)of the conductive portion 52 existing in an inner portion of the caprubber 50 has a relationship (S₁+S₂):S₁=100:5 to 15 in relation to atotal area (S₁+S₂) of the cross sectional area S₁ and a cross sectionalarea S₂ (a diagonally right-up hatching area in FIG. 3) of the caprubber 50. More specifically, the cross sectional area S₁ is equal to ormore than 5% and equal to or less than 15% in relation to the total area(S₁+S₂). Among them, 10% is preferable. If the total area is less than5%, a machining performance is deteriorated, and if the total area goesbeyond 15%, the volume of the conductive rubber is increased, and amotion performance and a wear resistance performance are significantlylowered. In the case that the conductive portion 52 enters into an innerportion of a strip rubber 53 mentioned later, the cross sectional areaS₁ of the conductive portion 52 here does not include a portion enteringinto the strip rubber 53.

As shown in FIG. 1 and FIG. 2, the strip rubber 53 is provided forcovering the side surface 50 a of the cap rubber 50. The strip rubber 53is famed of a rubber having a lower rubber hardness than the cap rubber50. The strip rubber 53 does not form the ground surface E. The rubberhardness here means a hardness which is measured according to adurometer hardness test (type A) of JISK6253. The conductive portion 52enters into an inner portion of the strip rubber 53. A side end portionof the cap rubber 50 forms a shoulder portion in a whole of the tire,and the shoulder portion tends to be deformed by an application of theload. The strip rubber 53 is lower in the rubber hardness than the caprubber 50, and can enhance the adhesion property to the cap rubber 50and the conductive portion 52 on the basis of softness of the striprubber 53. Further, since the conductive portion 52 enters into theinner portion of the strip rubber 53, it is possible to enhance theadhesion property of each of the rubbers (the conductive portion 52, thestrip rubber 53 and the cap rubber 50) in the shoulder portion. Further,since the strip rubber 53 does not come into contact with the roadsurface without forming the ground surface, it is possible to avoidreduction of endurance.

Further, as shown in FIG. 2, the cap rubber 50 is sectioned into acenter side cap rubber 50 c and an end side cap rubber 50 e by theconductive portion 52. A rubber hardness H_(50e) of the end side caprubber 50 e is equal to or lower than a rubber hardness H_(50c) of thecenter side cap rubber 50 c. The rubber hardness H_(50e) is higher thana rubber hardness H₅₃ of the strip rubber 53. A relationshipH_(50c)≥H_(50e)>H₅₃ is established. Among them, a relationshipH_(50c)>H_(50e)>H₅₃ is preferable. This is because a ground contactperformance is increased and the braking performance is improved by therubber hardness of the end side cap rubber 50 e. Further, since thestrain is reduced by the end side cap rubber 50 e, the strain isabsorbed in cooperation with the conductive portion 52, and thedurability is improved.

In the present embodiment, the conductive portion 52 is terminated inthe inner portion of the strip rubber 53 as shown in FIG. 4A. Asmentioned above, according to the SWOT structure, the rubber volume ofthe conductive portion 52 can be reduced. In an example in FIG. 4A, itis necessary that the strip rubber 53 is famed of the conductive rubberfor securing a conductive path. In addition, the side wall rubber 6, thecarcass layer 4 and the base rubber 51 can appropriately employ any oneof the conductive rubber and the nonconductive rubber. If the carcasslayer 4 is made of the conductive rubber, the side wall rubber 6 can bemade of the nonconductive rubber, and if the carcass layer 4 is made ofthe nonconductive rubber, the side wall rubber 6 may be made of theconductive rubber.

Other Embodiments

(1) As shown in FIG. 4B, the conductive portion 52 is terminated in theinner portion of the strip rubber 53 in the TOS structure. According tothe TOS structure as mentioned above, it is possible to avoiddeterioration of an outer appearance in comparison with the case thatthe conductive portion 52 passes through the strip rubber 53. In theexample in FIG. 4B, it is necessary that the strip rubber 53 and theside wall rubber 6 are formed of the conductive rubber for securing theconductive path. The other elements can appropriately employ any one ofthe conductive rubber and the nonconductive rubber.

(2) As shown in FIG. 5A, the conductive portion 52 may pass through thestrip rubber 53 and come into contact with the side wall rubber 6. Inthe example in FIG. 5A, the conductive path can be secured if the sidewall rubber 6 is made of the conductive rubber. In the case that theside wall rubber 6 is made of the nonconductive rubber, the strip rubber53 and the carcass layer 4 may be made of the conductive rubber.

(3) As shown in FIG. 5B, the strip rubber 53 may not be provided. Inthis case, since the conductive portion 52 comes into contact with theside wall rubber 6, the conductive path can be secured if the side wallrubber 6 is made of the conductive rubber.

EXAMPLES

In order to specifically show the structures and the effects of thepresent disclosure, the following evaluations were made with regard tothe following examples.

(1) Durability

A peeling force between the conductive portion 52 and the cap rubber 50(nonconductive rubber) was measured. The measurement was carried outaccording to JISK-3:1999. A peeling property was compared. Results ofevaluation of the peeling force was indicated by an index numberobtained with a result in a comparative example 1 defined as 100. Thegreater the numerical value is, the higher the adhesion property is andthe higher the endurance is.

(2) Braking Performance

Each of the tires was installed to a vehicle of a domestic sedan vehicle(2000 cc), a braking distance was measured when an anti-lock brakesystem (ABS) was actuated from a state of traveling on a road surface atthe speed of 100 km/h, and an inverse number of the measured value wascalculated. Evaluation was made by an index number obtained with aresult in the comparative example 1 defined as 100, indicating that thegreater the index number is, the more excellent the braking performanceis.

(3) Rolling Resistance

A rolling resistance was measured according to International StandardIS028580 (JIS D4234). Evaluation was made by an index number obtainedwith a result of the comparative example 1 defined as 100, indicatingthat the smaller the index number is, the lower the rolling resistanceis, and the more excellent the rolling resistance is.

Comparative Example 1

As shown in JP-A-2013-95323 and FIG. 6A, there was provided a curvedconductive portion 152 which reached the side surface 50 a of the caprubber 50 from the ground surface E. Four conductive portions 152 areconnected in the tire radial direction. The thickness of the conductiveportion 152 is fixed. The other structures of the comparative example 1were formed same as the example 1.

Comparative Example 2

As shown in FIG. 6B, there was provided a conductive portion 252 inwhich the number of the curved portions in FIG. 6A was set to one. Thethickness of the conductive portion 252 is fixed. The other structuresof the comparative example 2 were famed same as the example 1.

Example 1

As shown in FIG. 2, the conductive portion 52 was provided. Theconductive portion 52 is structured such that the thicknesses in theground surface and the side surface of the cap rubber are relativelysmall and the maximum thickness of the intermediate portion between theground surface and the side surface of the cap rubber is relativelylarge, and is famed into the crescent shape which is curved into theinner side in the tire width direction and the inner side in the radialdirection, in the tire meridian cross section.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 1 StructureFIG. 6A FIG. 6B FIG. 2 Thickness of Fixed Fixed Thickness of conductivecenter portion: portion large Thickness of both end portions: smallDurability 100 105 110 Braking 100 100 102 performance Rolling 100 95 90resistance

According to Table 1, it is known that the example 1 is more excellentthan the comparative examples 1 and 2 in all of the durability, thebraking performance and the rolling resistance.

The durability is improved in the comparative example 2 in comparisonwith the comparative example 1 because it is considered that thecomparative example 1 has four curved conductive portions but thecomparative example 2 has one curved conductive portion, so that thevolume of the conductive portion is reduced. The example 1 is improvedin comparison with the comparative example 2 because it is consideredthat the volume is not changed by the thickness change so much incomparison with the fixed thickness, however, the contact area of aninterface between the cap rubber and the conductive rubber is increased.

The following reason can be considered as the other reasons than theincrease in the contact area. The fixed thickness does not have aneffect of suppressing the defamation in grounding, however, thedefamation in grounding can be suppressed and the rolling resistance canbe reduced by the crescent shape which is thick in the center portionand is thin in both ends. The volume of the conductive rubber isincreased and the rolling resistance is deteriorated by the increase ofthe thickness in the center portion in the example 1 in comparison withthe comparative example 2, however, it is considered that the reductionin the rolling resistance due to the defamation suppression sufficientlyexceeds the deterioration of the rolling resistance.

With regard to the braking performance, it is considered that thecomparative examples 1 and 2 having the fixed thickness of theconductive portion do not have the effect of suppressing the deformationin the radial direction of the shoulder portion in grounding. On thecontrary, the deformation in the radial direction of the shoulderportion is suppressed in the example 1. As a result, it is consideredthat the braking performance is improved.

The rolling resistance is reduced in the comparative example 2 incomparison with the comparative example 1 because it is considered thatthe volume of the conductive portion is reduced. The rolling resistanceis further reduced in the example 1 in comparison with the comparativeexample 2 because of the following reason. The effect of suppressing thedefamation in grounding is not obtained in the case that the thicknessis fixed, however, the deformation in grounding can be suppressed andthe strain becomes smaller by the crescent shape which is thick in thecenter portion and thin in both ends. As a result, it is considered thatthe rolling resistance is reduced. The volume of the conductive rubberis somewhat increased by the increase of the thickness in the centerportion in the example 1 in comparison with the comparative example 2,however, the reducing amount of the rolling resistance on the basis ofthe shape sufficiently exceeds the volume increase.

As mentioned above, the pneumatic tire according to the presentembodiment has a cap rubber 50 which is famed of a nonconductive rubberand forms a ground surface E, and a conductive portion 52 which isprovided in at least one side end portion of a pair of side end portionsin both ends in a tire width direction of the cap rubber 50. Theconductive portion 52 is formed of a conductive rubber, and reaches aside surface 50 a of the cap rubber 50 from the ground surface E throughan inner portion of the cap rubber 50. The conductive portion 52 isrelatively small in a thickness D1 and D2 in the ground surface E and aside surface 50 a of the cap rubber 50, is relatively large in themaximum thickness D3 of an intermediate portion between the groundsurface E and the side surface 50 a of the cap rubber 50. The conductiveportion 52 is famed into a crescent shape which is curved into an innerside WD1 in a tire width direction and an inner side RD1 in a radialdirection, in a tire meridian cross section.

According to the structure, since the conductive portion 52 is famedinto the crescent shape which is curved into the inner side WD1 in thetire width direction and the inner side RD1 in the radial direction, andis relatively large in the maximum thickness of the intermediate portionin comparison with the end portion, the deformation extending in thetire radial direction is suppressed in comparison with the case that thethickness is fixed. Since the strain is reduced by the suppression ofthe defamation, the durability is improved. Further, the rollingresistance is reduced by the suppression of the defamation. Further,since the conductive portion 52 supports in relation to the radialdirection, the braking performance is improved.

According to the present embodiment, a cross sectional area S₁ of theconductive portion is equal to or more than 5% and equal to or less than15% in relation to a total area (S₁+S₂) of a cross sectional area S₁ ofthe conductive portion existing in the inner portion of the cap rubberand a cross sectional area S₂ of the cap rubber, in a half of the tiremeridian cross section.

According to the structure, it is possible to further secure themachining performance, the motion performance and the wear resistanceperformance.

According to the present embodiment, The tire has a strip rubber 53which covers a side surface 50 a of the cap rubber 50. The strip rubber53 is formed by a rubber which has a lower rubber hardness than the caprubber 50, and does not form any ground surface E. The conductiveportion 52 enters into an inner portion of the strip rubber 53.

According to the structure, since the strip rubber 53 is a soft rubberhaving a lower rubber hardness than the cap rubber 50, the adhesionproperty between the cap rubber 50 and the conductive portion 52 isenhanced on the basis of the softness even in the shoulder portion towhich the load tends to be applied, and it is possible to improve theendurance. Since the conductive portion 52 enters into the inner portionof the strip rubber 53, it is also possible to enhance the adhesionproperty of each of the rubbers in the shoulder portion and improve theendurance. Further, since the strip rubber 53 does not form the groundsurface E or come into contact with the road surface in spite of thelower rubber hardness than the cap rubber 50, it is possible to avoidthe deterioration of the endurance.

According to the present embodiment, the cap rubber 50 is sectioned intoa center side cap rubber 50 c and an end side cap rubber 50 e by theconductive portion. The rubber hardness of the end side cap rubber 50 eis lower than the rubber hardness of the center side cap rubber 50 c andis higher than the rubber hardness of the strip rubber 53.

According to the structure, the ground contact performance is increasedby the rubber hardness of the end side cap rubber 50 e, and the brakingperformance is improved. Further, since the end side cap rubber 50 e andthe conductive portion 52 cooperate and absorb the strain, thedurability is improved.

According to the present embodiment, the conductive portion 52 isterminated in the inner portion of the strip rubber 53.

According to the structure, the outer appearance is not deteriorated inthe TOS structure and the rubber volume can be reduced in the SWOTstructure.

According to the present embodiment, a display indicating an installingdirection of the tire to a vehicle is provided on an outer surface ofthe tire. The conductive portion 52 is provided at least in the side endportion which comes to an installation inner side to the vehicle.

According to the structure, the load tends to be applied to installationinner side in comparison with the installation outer side by setting thecamber angle, and the above effect becomes higher in comparison with thecase that the conductive portion 52 is provided only in the installationouter side.

The tread rubber 5 having the conductive portion 52 mentioned above canbe obtained easily by an extrusion molding method. In the extrusionmolding method, the tread rubber having a predetermined cross sectionalshape is formed into a band shape by a coextrusion of the cap rubber 50,the base rubber 51 and the conductive portion 52, and the annular treadrubber 5 is formed by jointing the end portions to each other.

It is possible to apply the structure employed in each of theembodiments to the other optional embodiment. The particular structureof each of the portions is not limited to the embodiments mentionedabove, but can be variously modified within a range which does notdeviate from the scope of the present invention.

What is claimed is:
 1. A pneumatic tire comprising: a cap rubber whichis formed of a nonconductive rubber and forms a ground surface; aconductive portion which is provided in at least one side end portion ofa pair of side end portions in both ends in a tire width direction ofthe cap rubber; and a strip rubber which covers a side surface of thecap rubber, wherein the conductive portion is formed of a conductiverubber, and reaches a side surface of the cap rubber from the groundsurface through an inner portion of the cap rubber, wherein theconductive portion is relatively small in a thickness in the groundsurface and a side surface of the cap rubber, is relatively large in themaximum thickness of an intermediate portion between the ground surfaceand the side surface of the cap rubber, and is formed into a crescentshape which is curved into an inner side in a tire width direction andan inner side in a radial direction, in a tire meridian cross section,wherein the strip rubber is formed of a conductive rubber which has alower rubber hardness than the cap rubber, and does not form any groundsurface, wherein the conductive portion enters into an inner portion ofthe strip rubber, and wherein the conductive portion is terminated inthe inner portion of the strip rubber.
 2. The pneumatic tire accordingto claim 1, wherein a cross sectional area S1 of the conductive portionis equal to or more than 5% and equal to or less than 15% in relation toa total area (S1+S2) of a cross sectional area S1 of the conductiveportion existing in the inner portion of the cap rubber and a crosssectional area S2 of the cap rubber, in a half of the tire meridiancross section.
 3. The pneumatic tire according to claim 1, wherein thecap rubber is sectioned into a center side cap rubber and an end sidecap rubber by the conductive portion, and wherein the rubber hardness ofthe end side cap rubber is lower than the rubber hardness of the centerside cap rubber and is higher than the rubber hardness of the striprubber.
 4. The pneumatic tire according to claim 1, wherein a displayindicating an installing direction of the tire to a vehicle is providedon an outer surface of the tire, and wherein the conductive portion isprovided at least in the side end portion which comes to an installationinner side to the vehicle.